Pinocytosis and degradation of exogenous proteins by cystinotic fibroblasts

Lysosomes of cystinotic human fibroblasts contain over 100-times the normal concentration of cystine. The high cystine concentration (probably in the millimolar range) might be expected to inhibit intralysosomal protein breakdown. A comparison of pinocytosis and degradation of five ¹²⁵I-labelled proteins (bovine serum albumin, formaldehyde-denatured bovine serum albumin, bovine pancreatic ribonuclease A and porcine lactate dehydrogenase isoenzymes H4 and M4) by human fibroblasts has been made, using one cystinotic and two normal cell-lines. The proteins each entered fibroblasts by adsorptive pinocytosis and were then degraded within the lysosomes by enzymes susceptible to leupeptin, the thiol-proteinase inhibitor. Each protein was captured by the fibroblasts at a characteristic rate, which was not different in cystinotic cells. Normal and cystinotic fibroblasts did not differ in their proteolytic capacity, as measured in extracts of disrupted cells. In intact fibroblasts, four of the five proteins were rapidly and fully digested following pinocytosis, in both cystinotic and normal cells. However, with formaldehyde-denatured albumin, the most resistant to degradation of the proteins tested, or with some other proteins in the presence of leupeptin, when the proteolytic capacity of lysosomes is diminished, intralysosomal degradation of pinocytosed protein was incomplete. Moreover, under these conditions, cystinotic cells demonstrated a lower rate of protein digestion than normal cells. It is concluded that pinocytic capture, rather than intralysosomal proteolysis, is commonly the rate-limiting step in the overall process of uptake and degradation of proteins by fibroblasts, and that intralysosomal cystine inhibits digestion of pinocytosed protein only in circumstances when degradation becomes the rate-limiting step.